Yeah, I wonder about the Vissians sometimes. If they’re “gone” for whatever reason, or if they’re (exponential growth of technology over time and all that) on another level by the 24th Century like the Metrons or whomever and don’t like to get involved in the going’s on of “younger races” (i.e. the Dominion War)
I always thought that the Vissians were second-wave Federation members. They weren't 2155 coalition-members with the Big Four, like the Orions and Denobulans were. But soon enough, news go around of this peaceful alliance of worlds, and more worlds expressed interest in joining. No reason to think the Vissians wouldn't have been among them.
And who knows? Perhaps, in time, their new associates were able to gently guide their society toward treating all three of its sexes more equitably.